Removal of metal ions from aqueous solution

ABSTRACT

A method of removing heavy metals from aqueous solution, a composition of matter used in effecting said removal, and apparatus used in effecting said removal. One or more of the polypeptides, poly (γ-glutamylcysteinyl)glycines, is immobilized on an inert material in particulate form. Upon contact with an aqueous solution containing heavy metals, the polypeptides sequester the metals, removing them from the solution. There is selectivity of poly (γ-glutamylcysteinyl)glycines having a particular number of monomer repeat units for particular metals. The polypeptides are easily regenerated by contact with a small amount of an organic acid, so that they can be used again to remove heavy metals from solution. This also results in the removal of the metals from the column in a concentrated form.

BACKGROUND OF THE INVENTION

This invention relates to biochemistry and environmental protection.This invention is the result of a contract with the Department of Energy(Contract No. W-7405-ENG-36).

Waste streams containing heavy metals, such as cadmium and copper, aregenerated in many industrial operations. The metals are toxic to animaland plant life and must be removed before the waste streams aredischarged into the environment. Also, there are many sites where watercontaining heavy toxic metals has been dumped; these sites must becleaned up or, at minimum, the sites must be stabilized to prevent thewaste from migrating to contaminate more of the environment.

Cadmium is an example of a toxic metal which must be excluded from theenvironment, it accumulates readily in living systems and, in humans,has been implicated as the cause of renal disturbances, lunginsufficiency, bone lesions, hypertension, and cancer. The U.S.Environmental Protection Agency limits the amount of cadmium which maybe present in drinking water to 10 parts per billion. Cadmium containingwaste streams are generated in such industrial operations as zincrefining, battery manufacturing, electroplating, and pigmentmanufacturing.

BRIEF DESCRIPTION OF THE INVENTION

This invention is a method of removing heavy metals from aqueoussolution, a composition of matter used in effecting said removal, andapparatus used in effecting said removal. One or more of thepolypeptides, poly (γ-glutamylcysteinyl)glycines, is immobilized on aninert material in particulate form. Upon contact with an aqueoussolution containing heavy metals, the polypeptides sequester the metals,removing them from the solution. There is selectivity of poly(γ-glutamylcysteinyl)glycines having a particular number of monomerrepeat units for particular metals. The polypeptides are easilyregenerated by contact with a small amount of an organic acid, so thatthey can be used again to remove heavy metals from solution. This alsoresults in the removal of the metals from the column in a concentratedform.

In a broad embodiment, the present invention is a method of removingheavy metals from an aqueous solution comprising contacting an aqueoussolution with a solid substance comprised of water-insoluble polymericmaterial to which is attached molecules of poly(γ-glutamylcysteinyl)glycines for a time period effective for metals tobecome attached to said poly (γ-glutamylcysteinyl)glycines andseparating said aqueous solutions, which is depleted of metals, fromsaid solid substance.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows the structural formula of a poly(γ-glutamylcysteinyl)glycine having two monomer repeat units.

DETAILED DESCRIPTION OF THE INVENTION

A poly (γ-glutamylcysteinyl)glycine molecule of this invention is apolypeptide which consists of a chain of monomer repeat units having theamino acid glycine attached to it. Each monomer repeat unit consists oftwo amino acids, glutamate and cysteine, joined by a peptide, or gamma,bond. The poly (γ-glutamylcysteinyl)glycines are frequently representedby (Glu-Cys)nGl, where n is equal to the number of monomer repeat units.In experimentation associated with this invention, poly(γ-glutamylcysteinyl)glycines having 2, 3, 4, and 5 repeat units havebeen produced and it is expected that the number of repeat units whichmay be utilized in the practice of this invention will range up to 10 ormore.

The poly (γ-glutamylcysteinyl)glycines of this invention have anaffinity for the heavy toxic metals cadmium, copper, and zinc and willcombine with these metals in water solution. In addition to these threemetals, which have been the subject of experimentation, it is expectedthat poly (γ-glutamylcysteinyl)glycines will also have an affinity forlead, mercury, and nickel. Certain poly (γ-glutamylcysteinyl)glycinesare selective for particular metals. It has been demonstrated byexperimentation that the poly (γ-glutamylcysteinyl)glycine having tworepeat units will tend to sequester cadmium in preference to the othermetals. Also, the compound having three repeat units favors copper. Itis expected that additional selectivities exist, such that knowledge ofthe aqueous stream to be treated will allow a particular poly(γ-glutamylcysteinyl)glycine to be selected. If two metals are to beremoved, two or more poly (γ-glutamylcysteinyl)glycines may be used. Onthe other hand, it may be desirable to remove only one metal from astream containing two or more metals. In this case the poly(γ-glutamylcysteinyl)glycine which is selective for that metal would beused.

Since the poly (γ-glutamylcysteinyl)glycines are soluble in water, theymust be immobilized, or made insoluble. This is done by attaching thepoly (γ-glutamylcysteinyl)glycine molecules to an inert polymericmaterial, which is usually in the form of beads, which may range from0.01 to 20 mm or more in diameter. Any relatively inert polymer whichhas chemical sites to which will attach the free amino group of theglutamate group on one end of the chain may be used. A polysaccharidehas been used in the experimentation.

The solid substance consisting of the polymeric material with the poly(γ-glutamylcysteinyl)glycines attached is placed in a container, such asan elongated vertical cylindrical vessel, and the aqueousmetal-containing solution is allowed to flow through the solid substanceby gravity. When the solid substance has reached its capacity to absorbmetals, it is regenerated, that is, prepared for reuse in removingmetals, by passing a small quantity of a high molecular weight organicacid through it. For example, 3 ml of oxalic acid was sufficient toregenerate a column which had removed metals from 1000 ml of solution.Large organic oxides having a low pH and weak chelating properties, suchas citric acid and maleic acid, may be used.

The polypeptides of this invention may, conceivably, be prepared bychemical means, but are most advantageously prepared by culturingcertain substances in the presence of a heavy metal. Though a particularmetal is used to stimulate production of poly(γ-glutamylcysteinyl)glycines, the material produced has affinities forother metals, as described above.

The following paragraphs describe a portion of the experimentation whichwas accomplished.

Suspension cell cultures of Datura innoxia have been selected forresistance to different concentrations of CdCl₂ using a stepwiseselection protocol. Variant cell lines retain the ability to grow innormally toxic concentrations of Cd after growth in its absence for morethan seven hundred generations. Resistance to Cd correlates withsynthesis of large amounts of small cysteine-rich Cd complexes.

Cadmium tolerant Datura innoxia cell cultures are grown in a standardplant cell culture media containing 250 μm CdCl₂ for at least 48 hours,with shaking at 30° C. Growth under these conditions results in thesynthesis of large amounts of poly(γ-glutamylcysteinyl)glycines, whichaccumulates within the cells.

To extract the polypeptides, the cells are washed once in an ice-coldbuffer containing 10 mM Tris-HCl, pH 7.4, 10 mM KCl, 1.5 mM MgCl₂ and 20mM 2-mercaptoethanol. Cells are collected from the buffer bycentrifugation at low speed for 10 minutes, resuspended in the samebuffer, and broken open with a homogenizer. The resulting extract iscentrifuged for 15 minutes at 15,000×g to remove insoluble material andthe resulting supernatant is passed through a Sephadex G-50 (fine)column, which separates the polypeptides from the majority of theremaining cellular material. Fractions collected from the column whichcontain the polypeptides are identified by the presence of boundcadmium. These fractions are pooled and the polypeptides are washed andconcentrated by ultrafiltration. The resulting preparation is used as asource of poly(γ-glutamylcysteinyl)glycines to be attached to Sepharosebeads.

The amount of polypeptide present is determined by assaying for theamount of sulthydryl groups present using an Ellman's reaction.Sepharose 4B (Sigman Chemical Co., St. Louis, Mo.) is washed in a largeexcess of triple distilled water. The hydrated Sepharose is thensuspended in an equal volume of 5 M potassium phosphate solution andchilled on ice. 0.4 volumes of a solution containing 100 mg/ml CNBr(cyanogen bromide) in distilled water is added dropwise over a period oftwo minutes. The suspension is shaken gently during the addition of theCNBr and is allowed to react for eight minutes on ice.

The CNBr-activated Sepharose is then washed with five volumes of asolution containing 0.25 M NaHCO₃ pH 9.0.

The CNBr-activated Sepharose is then mixed with a solution containing aknown quantity of poly(γ-glutamylcysteinyl)glycine. This suspension isallowed to react overnight at room temperature.

The above suspension is then poured into a glass or plastic columncontaining a glass frit in the bottom which allows passage of liquid butnot the metal-binding matrix. As the liquid drips from the column (itcan be removed using gravity flow or by pumping), the matrix is washedby addition of at least five volumes of solution containing 0.25 MNaHCO₃, pH 9.0. This is followed by washing the column with five volumesof the same solution containing 1 M NaCl and five volumes of a solutioncontaining 1 M ethanolamine, pH 9.0 (pH adjusted with HCl). The columncan be stored in this latter solution, then washed again with 0.25 MNaHCO₃, followed by distilled water.

The column is then washed with five volumes of a solution containing 0.1M oxalic acid. This removes the cadmium which was bound by thepolypeptides while the polypeptides were still within the cell. It alsofrees the binding sites of the polypeptides for binding more metal ion.

A stock solution containing 5 μM CdCl₂ containing a small amount ofradioactive ¹⁰⁹ Cd is passed through the column and fractions arecollected from the bottom of the column and assayed for the presence ofthe radioactive cadmium. No radioactive cadmium can be detected untilall of the metal-binding sites on the column are saturated. Knowledge ofthe specific gravity of the cadmium solution allows determination of theamount of cadmium bound by a specific batch of metal-binding matrix.Cadmium bound to the column is removed by again washing the column witha small volume of a solution containing 0.1 M oxalic acid. This washreleases the cadmium ion from the column in a concentrated solution andresults in the regeneration of cadmium binding sites on the column. Theend result is a significant concentration of the cadmium from thebeginning aqueous solution.

The polypeptides were first identified by their ability to bindradioactive cadmium. Extracts from Cd-tolerant Datura innoxia cells werepassed through a Sephadex G-50 column which separates molecules basedprimarily on size. Fractions from the columns were assayed for cadmiumand it was found that certain fractions contained greater than 90% ofthe cadmium known to be within the cells. These fractions were collectedand further purified by affinity chromatography on Thiopropyl Sepharose6B, which binds compounds which are rich in cystein (we knew from aminoacid labeling experiments that cysteine was one of the major componentsof the cadmium binding fractions). It was found that the components ofthe fractions contained only three different amino acids (by amino acidanalysis of the purified material); cysteine, glutamate and glycine in aratio close to 3:3:1. We attempted to sequence the polypeptides usingEdman degradation, but this did not result in the degradation of thepolypeptides, suggesting that either the peptide bonds were blocked orwere not linked to one another via the α-carboxyl group. We sequencedthe polypeptides using a combination of enzymatic degradation of thepolypeptides, starting from the carboxyl end. The presence ofγ-carboxamide linkages was first suggested by the stability of thepolypeptides to Edman degradation and was confirmed by ¹³ C NMR.

Experimentation was conducted at ambient temperatures, but thepolypeptides are stable over a temperature range of at least 4° to 85°C. Selectivity of the polypeptide may vary with temperature; this wouldbe useful in designing a commercial process. The Sepharose used in theexperiments had a hydrated size of from 60 to 140 microns.

                  TABLE                                                           ______________________________________                                        Cd concentration,                                                                           Bound Cd,                                                       micromolar    moles Cd/mole polypeptide                                       ______________________________________                                        1             0.2                                                             5             1.41                                                            20            1.66                                                            47.5          1.91                                                            100           2.01                                                            ______________________________________                                    

The Table shows the results of a series of experiments in which aqueoussolutions having varying Cd concentrations were passed through a column2.5 cm I.D.×20 cm long containing an estimated 12.45 mg of poly(γ-glutamylcysteinyl)glycines on Sepharose. The column was regeneratedafter each of the 5 solutions was passed through it. A small amount ofradioactive Cd was present in each solution in order to determine thetotal amount of Cd bound by the column. It can be seen that the Cdcapacity of the column was dependent on the concentration of Cd in thesolution.

What is claimed is:
 1. A method of removing heavy metals from an aqueoussolution comprising contacting an aqueous solution with a solidsubstance comprised of water-insoluble polymeric material to which isattached molecules of poly (γ-glutamylcysteinyl)glycines for a timeperiod effective for metals to become attached to said poly(γ-glutamylcysteinyl)glycines and separating said aqueous solution,which is depleted of metals, from said solid substance.
 2. The method ofclaim 1, where the molecular weight of the particular poly(γ-glutamylcysteinyl)glycine used in said method is established byreference to which particular metal is to be removed from said aqueoussolution.
 3. The method of claim 1, where the number of monomer repeatunits of said poly (γ-glutamylcysteinyl)glycines is from 2 to about 10.4. The method of claim 1, where said metals are cadmium, copper, zinc,mercury, lead, and nickel.
 5. The method of claim 1, where saidpolymeric material is in the form of spheres having a diameter rangingfrom about 0.01 to 20 mm.
 6. The method of claim 1, where said polymericmaterial is a polysaccharide.
 7. The method of claim 1, where said poly(γ-glutamylcysteinyl)glycine molecules have substantially the samemolecular weight.
 8. The method of claim 1, where the number of monomerrepeat units of said poly (γ-glutamylcysteinyl)glycine is two and themetal to be removed from said aqueous solution is cadmium.
 9. The methodof claim 1, where the number of monomer repeat units of said poly(γ-glutamylcysteinyl)glycine is three and the metal to be removed fromsaid aqueous solution is copper.
 10. The process of claim 1, furtherincluding regenerating said solid substance by contacting said solidsubstance with a high molecular weight organic acid for a time periodeffective for said metals to go into solution.
 11. The process of claim10, where said large organic acid is oxalic acid.